TITLE: GCN CIRCULAR NUMBER: 33315 SUBJECT: GRB 230209B: Swift/BAT-GUANO detection outside the coded FOV DATE: 23/02/11 00:37:33 GMT FROM: Aaron Tohuvavohu at U Toronto Gayathri Raman (PSU), Jamie A. Kennea (PSU), James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto) report: Swift/BAT did not localize GRB 230209B onboard (T0: 2023-02-09T22:32:36 UTC, Fermi/GBM GCN 33310, INTEGRAL trig 10198). The Fermi and INTEGRAL notices, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst with a sqrt(TS) of 42.1 in a 2.048 s analysis time bin. The burst duration as seen by BAT is ~60 s. NITRATES results indicate a burst coming from outside the FOV, with DeltaLLHOut of -53. The NITRATES best fit OFOV position is consistent with the Fermi/GBM localization (GCN 33310). See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut. GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/